Sometimes it is the only position left in the family. As a new child enters the family and grows into it he has to find where he fits. The parents have the Parent and Adult ego states covered and a sibling may have the outspoken, demanding position taken so the ‘happy to help’ good child position is the only one left.
First, this is better for the parents (which in turn is better for the children) because there is usually more support readily available. Second, from an attachment theory point of view it is better if children are raised in a polymatric family rather than a monomatric family.
The therapeutic response is clear. Get the family secret exposed and this can change family dynamics, quite profoundly at times.